GLEP: 59
Title: Manifest2 hash policies and security implications
Version: $Revision: 1.4 $
Last-Modified: $Date: 2010/01/13 03:26:53 $
Author: Robin Hugh Johnson ,
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Requires: 44
Created: October 2006
Updated: November 2007, June 2008, July 2008, October 2008, January 2010
Updates: 44
Post-History: December 2009
Abstract
========
While Manifest2 format allows multiple hashes, the question of which
checksums should be present, why, and the security implications of such
have never been resolved. This GLEP covers all of these issues, and
makes recommendations as to how to handle checksums both now, and in
future.
Motivation
==========
This GLEP is being written as part of the work on signing the Portage
tree, but is only tangentially related to the actual signing of
Manifests. Checksums present one possible weak point in the overall
security of the tree - and a comprehensive security plan is needed.
Specification
=============
The bad news
------------
First of all, I'd like to cover the bad news in checksum security.
A much discussed point, as been the simple question: What is the
security of multiple independent checksums on the same data?
The most common position (and indeed the one previously held by myself),
is that multiple checksums would be an increase in security, but we
could not provably quantify the amount of security this added.
The really bad news, is that this position is completely and utterly
wrong. Many of you will be aghast at this. There is extremely little
added security in multiple checksums [J04]. For any set of checksums,
the actual strength lies in that of the strongest checksum.
How fast can MD5 be broken?
---------------------------
For a general collision, not a pre-image attack, since the original
announcement by Wang et al [W04], the time required to break MD5 has
been massively reduced. Originally at 1 hour on a near-supercomputer
(IBM P690) and estimated at 64 hours with a Pentium-3 1.7Ghz. This has
gone down to less than in two years, to 17 seconds [K06a]!
08/2004 - 1 hour, IBM pSeries 690 (32x 1.7Ghz POWER4+) = 54.4 GHz-Hours
03/2005 - 8 hours, Pentium-M 1.6Ghz = 12.8 Ghz-Hours
11/2005 - 5 hours, Pentium-4 1.7Ghz = 8.5 Ghz-Hours
03/2006 - 1 minute, Pentium-4 3.2Ghz = .05 Ghz-Hours
04/2006 - 17 seconds, Pentium-4 3.2Ghz = .01 Ghz-Hours
If we accept a factor of 800x as a sample of how much faster a checksum
may be broken over the course of 2 years (MD5 using the above data is
>2000x), then existing checksums do not stand a significant chance of
survival in the future. We should thus accept that whatever checksums we
are using today, will be broken in the near future, and plan as best as
possible. (A brief review [H04] of the present SHA1 attacks indicates an
improvement of ~600x in the same timespan).
And for those that claim implementation of these procedures is not yet
feasible, see [K06b] for an application that can produce two
self-extracting .exe files, with identical MD5s, and whatever payload
you want.
The good news
-------------
Of the checksums presently used by Manifest2, one stands close to being
completely broken: SHA1. The SHA2 series has suffered some attacks, but
still remains reasonably solid [G07],[K08]. No attacks against RIPEMD160
have been published, however it is constructed in the same manner as
MD5, SHA1 and SHA2, so is also vulnerable to the new methods of
cryptanalysis [H04].
To reduce the potential for future problems and any single checksum
break leading to a rapid decrease in security, we should incorporate the
strongest hash available from each family of checksums, and be prepared
to retire old checksums actively, unless there is a overriding reason to
keep a specific checksum.
What should be done
-------------------
Portage should always try to verify all supported hashes that are
available in a Manifest2, starting with the strongest ones as maintained
by a preference list. Over time, the weaker checksums should be removed
from Manifest2 files, once all old Portage installations have had
sufficient time to upgrade. We should be prepared to add stronger
checksums wherever possible, and to remove those that have been
defeated.
An unsupported hash is not considered to be a failure unless no
supported hashes are available.
Checksum depreciation
~~~~~~~~~~~~~~~~~~~~~
For the current Portage, SHA1 should be gradually removed, as presents
no advantages over SHA256. Beyond one specific problem (see the next
paragraph), we should add SHA512 (SHA2, 512 bit size), the Whirlpool
checksum (standardized checksum, with no known weaknesses). In future,
as stream-based checksums are developed (in response to the development
by NIST [AHS]), they should be considered and used.
There is one temporary stumbling block at hand - the existing Portage
infrastructure does not support SHA384/512 or Whirlpool, thus hampering
their immediate acceptance. SHA512 is available in Python 2.5, while
SHA1 is already available in Python 2.4. After Python2.5 is established
in a Gentoo media release, that would be a suitable time to remove SHA1
from Manifest2 files.
Backwards Compatibility
=======================
Old versions of Portage may support and expect only specific checksums.
This is accounted for in the checksum depreciation discussion.
References
==========
[AHS] NIST (2007). "NIST's Plan for New Cryptographic Hash Functions",
(Advanced Hash Standard). http://csrc.nist.gov/pki/HashWorkshop/
[BOBO06] Boneh, D. and Boyen, X. (2006). "On the Impossibility of
Efficiently Combining Collision Resistant Hash Functions"; Proceedings
of CRYPTO 2006, Dwork, C. (Ed.); Lecture Notes in Computer Science
4117, pp. 570-583. Available online from:
http://crypto.stanford.edu/~dabo/abstracts/hashing.html
[H04] Hawkes, P. and Paddon, M. and Rose, G. (2004). "On Corrective
Patterns for the SHA-2 Family". CRYPTO 2004 Cryptology ePrint Archive,
Report 2004/204. Available online from:
http://eprint.iacr.org/2004/207.pdf
[J04] Joux, Antoie. (2004). "Multicollisions in Iterated Hash
Functions - Application to Cascaded Constructions;" Proceedings of
CRYPTO 2004, Franklin, M. (Ed); Lecture Notes in Computer Science
3152, pp. 306-316. Available online from:
http://web.cecs.pdx.edu/~teshrim/spring06/papers/general-attacks/multi-joux.pdf
[K06a] Klima, V. (2006). "Tunnels in Hash Functions: MD5 Collisions
Within a Minute". Cryptology ePrint Archive, Report 2006/105.
Available online from: http://eprint.iacr.org/2006/105.pdf
[K06b] Klima, V. (2006). "Note and links to high-speed MD5 collision
proof of concept tools". Available online from:
http://cryptography.hyperlink.cz/2006/trick.txt
[K08] Klima, V. (2008). "On Collisions of Hash Functions Turbo SHA-2".
Cryptology ePrint Archive, Report 2008/003. Available online from:
http://eprint.iacr.org/2008/003.pdf
[G07] Gligoroski, D. and Knapskog, S.J. (2007). "Turbo SHA-2".
Cryptology ePrint Archive, Report 2007/403. Available online from:
http://eprint.iacr.org/2007/403.pdf
[W04] Wang, X. et al: "Collisions for Hash Functions MD4, MD5,
HAVAL-128 and RIPEMD", rump session, CRYPTO 2004, Cryptology ePrint
Archive, Report 2004/199, first version (August 16, 2004), second
version (August 17, 2004). Available online from:
http://eprint.iacr.org/2004/199.pdf
Thanks to
=========
I'd like to thank the following folks, in no specific order:
- Ciaran McCreesh (ciaranm) - for pointing out the Joux (2004) paper,
and also being stubborn enough in not accepting a partial solution.
- Marius Mauch (genone), Zac Medico (zmedico) and Brian Harring
(ferringb): for being knowledgeable about the Portage Manifest2
codebase.
Copyright
=========
Copyright (c) 2006-2010 by Robin Hugh Johnson. This material may be
distributed only subject to the terms and conditions set forth in the
Open Publication License, v1.0.
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